This invention relates to a system and method for verifying the movement of an amount of fluid through a fluid delivery probe and/or for detecting a fluid surface within a container that is entered by the fluid delivery probe.
Automated analyzers are commonly used by clinical laboratories and in health science research to assay and determine inter alia the presence or amount of a particular analyte or group of analytes in a biological sample. Typical biological samples for assaying include blood, urine, cerebrospinal fluid, pus, seminal fluid, sputum, stool, plants, water and soil. Analytes commonly targeted in biological samples include antibodies, antigens, nucleic acids, toxins and other chemicals. Clinicians especially prefer automated analyzers over manual procedures because of their high-throughput capabilities, reduced labor expenses, and the limits they place on human error that can lead to false or misleading results. To be most useful, an analyzer preferably automates both the sample preparation and sample processing steps of an assay.
Sample preparation may be initiated by an automated fluid transfer system which transfers a fluid sample from a sample container to a reaction vessel for analysis. The automated fluid transfer system may also be used to transfer one or more assay reagents from their respective containers or associated reservoirs into the sample-holding reaction vessel. After conducting the appropriate sample processing steps for a given assay, the contents of the reaction vessel may be examined by the automated analyzer to determine the presence or amount of at least one specifically targeted analyte. Detecting a targeted analyte in the sample might provide an indication that a particular pathogenic organism is present in the sample, or it might indicate a specific disease condition or state useful for determining or adapting a treatment regimen.
The fluid transfer system typically includes a fluid delivery probe operatively carried on a robotically controlled arm to perform aspiration and dispensing functions required for the transfer process and a pump coupled to the probe by a conduit system. During a fluid transfer operation, the robotic arm, under the command of a system controller, positions the fluid delivery probe above a sample or reagent container and moves the probe into the container until the tip of the probe reaches the fluid surface in the container. It is desirable that the distal tip of the probe be maintained right at the fluid surface to avoid ingesting air into the probe during aspiration and to avoid possible cross-contamination that can occur if the probe is unnecessarily submerged into the fluid and fluid residue is carried on the exterior of the probe from one sample to another. Accordingly, a desirable feature of an automated fluid delivery probe is a means by which contact of the probe tip with the fluid surface can be detected as the probe is being lowered into a fluid-containing vessel.
With the probe tip maintained at the fluid surface, a pump, such as a syringe type pump, is activated to draw an amount of sample or reagent fluid from the container into the probe. The amount of fluid aspirated will correspond to the volume and number of aliquots to be dispensed from the probe. The fluid delivery probe is thereafter moved into a position above a reaction vessel and a precise aliquot of fluid is dispensed. To ensure that accurate results are obtained in the tests, a predetermined volume of the sample must be accurately aspirated and dispensed into the reaction vessel. Accordingly, another desirable feature of an automated fluid delivery probe is automated verification of fluid dispensed from the probe.
Different devices and methods for automatically determining when a probe tip has contacted a fluid surface in a container have been proposed in the available literature. For example, some surface detection sensors operate on the basis of capacitance. The probe, if made from a conductive, e.g., metal, conduit, will exhibit a finite amount of electrical capacitance. When the probe tip contacts a fluid surface, the higher dielectric constant and greater surface area of the fluid results in a small, but measurable, increase in the capacitance of the probe.
Other surface detection mechanisms for incorporation onto a fluid delivery probe include two or more electrodes which may comprise tubular elements arranged coaxially with each other (see, e.g., U.S. Pat. Nos. 5,304,347 and 5,550,059) or elongated conductors extending along the length of the probe and arranged in a spaced, parallel relationship (see, e.g., U.S. Pat. Nos. 5,045,286 and 5,843,378). When the probe contacts a fluid surface, the fluid, which contacts both electrodes simultaneously, electrically couples the electrodes to each other. If a voltage is applied across the electrodes the electrical coupling caused by the electrodes contacting the fluid surface results in a measurable change in the voltage drop across the electrodes.
U.S. Pat. Nos. 5,013,529 and 5,665,601 describe surface detection devices which incorporate a pressure sensor connected to a fluid line through which constant pressure gas is expelled through the tip of the probe. When the tip contacts the fluid surface, thereby blocking the gas emitting orifice (i.e., the end opening of the probe), a measurable change in the pressure is exhibited. U.S. Pat. No. 6,100,094 describes a surface detection device which includes an optic emitter which emits light axially through, or alongside, a tip. The light is reflected from the fluid surface back into the tip to a light sensor disposed within the tip. The amount of light reflected back to the light sensor detectably changes when the tip contacts the fluid surface.
The prior art surface detection sensors described above each suffer from certain shortcomings. For example, achieving adequate accuracy and repeatability with capacitive surface sensors can be difficult because the change in capacitance exhibited when a probe contacts a fluid surface can be very small and thus difficult to detect. This is especially true where the fluid is a conductive fluid with a low dielectric value. Furthermore, because of the small capacitance changes exhibited, capacitive surface detection sensors can be susceptible to inaccuracies due to fluctuating stray capacitances caused by adjacent moving structures or changes in the amount of fluid contained in the probe and/or container.
Dual electrode surface detection devices constructed to date, with side-by-side or coaxial arrangement of the electrodes, are complex and cumbersome. Surface detection devices that emit constant pressure gas can cause disturbances and even bubbling and/or atomization of the fluid. The effectiveness of optic sensors can be diminished due to residue or other buildup on the optic emitter and/or receiver.
Other devices and methods are described in the available literature for verifying aspiration and/or delivery of a fluid from the probe. For example, U.S. Pat. No. 6,121,049 describes a system wherein the pressure needed to hold up a column of aspirated fluid in the probe can be measured and compared to a predetermined standard to determine if a proper amount of fluid has been aspirated. By verifying a proper aspiration, a proper subsequent fluid delivery can, theoretically, be inferred. U.S. Pat. No. 5,559,339 describes a system which includes optical sensors, each with an emitter-receiver pair, disposed adjacent the pipette tip. Fluid flowing from the tip breaks the electromagnetic beam between the emitter and receiver, thereby indicating the flow of fluid. The duration of fluid flow can be monitored to determine if a proper amount of fluid has been dispensed.
Such fluid flow verification devices suffer from shortcomings which can limit their effectiveness. Pressure sensors that measure the amount of pressure required to hold up a column of aspirated fluid may be effective for confirming a proper aspiration of fluid, but, because fluid delivery can be interrupted by system leaks or occlusions blocking the probe, such sensors do not necessarily provide confirmation of proper fluid delivery. Furthermore, such devices are useful only for fluid delivery procedures that involve aspiration of fluid into the probe prior to delivery of the fluid from the probe into a reaction vessel. Such devices will not provide confirming information for fluid transfer systems in which fluid is pumped directly from a reservoir through the fluid delivery probe and into a reaction vessel without first being aspirated from another container.
As with surface detection devices that employ optic emitters and receivers, the effectiveness of the optic sensors employed to verify fluid flow can be diminished by residual build-up or other debris interfering with the emission or reception of the electromagnetic beam.
Accordingly the devices and methods described heretofore in the prior art are susceptible to further improvement. Moreover, although surface detection and fluid delivery verification are important features of a consistently accurate automated fluid delivery probe, the prior art does not describe a simple, effective, and accurate method and device for providing the combined capabilities of surface detection and fluid delivery verification in a single fluid delivery probe. Finally, the prior art does not describe a fluid delivery verification method or device in which secondary, redundant means are employed for verifying fluid delivery to guard against erroneous indications of proper fluid delivery.